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This paper reports the influence of dysprosium ion (Dy³⁺) substitution on the structural and magnetic properties of NiDy<i>_x</i>Fe_{2&#8722;<i>x</i>}O₄ (0.0 &#8804; <i>x</i> &#8804; 0.1) nanoparticles (NPs) prepared using a hydrothermal method. The structure and morphology of the as-synthesized NPs were characterized via X-ray diffraction (XRD), scanning and transmission electron microscope (SEM, and TEM) analyses. ⁵⁷Fe M&#246;ssbauer spectra were recorded to determine the Dy³⁺ content dependent variation in the line width, isomer shift, quadrupole splitting, and hyperfine magnetic fields. Furthermore, the magnetic properties of the prepared NPs were also investigated by zero-field cooled (ZFC) and field cooled (FC) magnetizations and AC susceptibility measurements. The M_ZFC (T) results showed a blocking temperature (<i>T</i>_B). Below <i>T</i>_B, the products behave as ferromagnetic (FM) and act superparamagnetic (SPM) above <i>T</i>_B. The M_FC (<i>T</i>) curves indicated the existence of super-spin glass (SSG) behavior below <i>T</i>_s (spin-glass freezing temperature). The AC susceptibility measurements confirmed the existence of the two transition temperatures (i.e., <i>T</i>_B and <i>T</i>_s). Numerous models, e.g., Neel&#8722;Arrhenius (N&#8722;A), Vogel&#8722;Fulcher (V&#8722;F), and critical slowing down (CSD), were used to investigate the dynamics of the systems. It was found that the Dy substitution enhanced the magnetic interactions.